Traditionally it has been a common practice for locomotive designers to limit the creepage or slip of the wheels on the rail to approximately one percent in order to avoid the condition of total wheel slip which results in rail or wheel burn. It was determined later that the wheel/rail contact has considerably more unused capacity to produce adhesion beyond one percent creepage.
An improvement in the locomotive adhesion and traction capacity was achieved in a second generation control system called the creepage control. In this approach, controlled wheel slip is provided in a way which limits the maximum wheel slip and yet allows sufficient slippage for the wheels to obtain high adhesion. This method is a significant improvement on the earlier practice and considerably high adhesion levels have been achieved by using this approach. It has, however, several major disadvantages. These are:
(1) Since somewhat arbitrary creepage limits are used for limiting the power in the motors, the wheels end up operating in an adhesion creepage domain which is often unstable, resulting in both mechanical and electrical instabilities in the system. This will be more clear in later discussions. Therefore, all elements or members of the drive or power train starting from the wheels all the way up to the engine including the electrical power plant have to sustain sudden changes and shock loading. This can lead to early failure of the weaker links in the drive chain.
(2) The adhesion levels achieved in this method are quite high but not in the optimum region on a continuous basis. In other words, improvements in adhesion levels are still possible.
(3) Creepages larger than necessary lead to wasted energy and fuel consumption. They also produce higher wheel and rail wear than is necessary for producing the operating adhesion levels.
(4) There is one other problem with the present control systems. They resort to application of sand between the wheel and the rail when insufficient adhesion is being produced. There is no indication of the need for sand application given considerably in advance. Such indication, if it were available, could enable a rail cleaning system such as in U.S. Pat. No. 4,781,121 to start operating and cleaning the rail. This would allow sand application to be avoided in most instances, which is desirable because sand increases wheel/rail wear by ten to one hundred times (see Kumar, S., Krishnamoorthy, P. K. and Prasanna Rao, D. L., "Wheel-Rail Wear and Adhesion With and Without Sand for a North American Locomotive", A.S.M.E. Journal of Engineering for Industry, May 1986, Vol. 108, pp. 141-147).
The present invention overcomes all the above four difficulties. It helps to operate the locomotive wheels in the stable adhesion/creepage domain. It permits increase of power/generator excitation and wheel creep only up to certain maximum values which depend on the wheel/rail contact characteristics and by which the fuel consumption is kept low as well as the wheel/rail wear is kept low. It achieves nearly optimum levels of adhesion on a continuous basis. It achieves a clear signal in advance that rail cleaning is needed and activates the cleaning system for increased adhesion levels. It also activates sand application when creep exceeds a certain specified high value and shuts it off when it is not needed. These factors will become more clear in the later discussion with the use of figures.